Coupling spin 'clock states' to superconducting circuits

A central goal in quantum technologies is to maximize GT2, where G stands for the rate at which each qubit can be coherently driven and T2 is the qubit's phase coherence time. This is challenging, as increasing G (e.g. by coupling the qubit more strongly to external stimuli) often leads to deleterious effects on T2. Here, we study a physical situation in which both G and T2 can be simultaneously optimized. We measure the coupling to microwave superconducting coplanar waveguides of pure (i.e. non magnetically diluted) crystals of HoW10 magnetic clusters, which show level anticrossings, or spin clock transitions, at equidistant magnetic fields. The absorption lines give a complete picture of the magnetic energy level scheme and, in particular, confirm the existence of such clock transitions. The quantitative analysis of the microwave transmission allows monitoring the overlap between spin wave functions and gives information about their coupling to the environment and to the propagating photons. The formation of quantum superpositions of spin-up and spin-down states at the clock transitions allows simultaneously maximizing the spin-photon coupling and minimizing environmental spin perturbations. Using the same experimental device, we also explore the coupling of these qubits to a 11.7 GHz cavity mode, arising from a nonperfect microwave propagation at the chip boundaries and find a collective spin to single photon coupling GN = 100 MHz. The engineering of spin states in molecular systems offers a promising strategy to combine sizeable photon-mediated interactions, thus scalability, with a sufficient isolation from unwanted magnetic noise sources.Comment: 7 pages, 5 figure

Optimal coupling of Ho W<sub>10 molecular magnets to superconducting circuits near spin clock transitions

A central goal in quantum technologies is to maximize GT2, where G stands for the coupling of a qubit to control and readout signals and T2 is the qubit’s coherence time. This is challenging, as increasing G (e.g., by coupling the qubit more strongly to external stimuli) often leads to deleterious effects on T2. Here, we study the coupling of pure and magnetically diluted crystals of Ho W10 magnetic clusters to microwave superconducting coplanar waveguides. Absorption lines give a broadband picture of the magnetic energy level scheme and, in particular, confirm the existence of level anticrossings at equidistant magnetic fields determined by the combination of crystal field and hyperfine interactions. Such “spin clock transitions” are known to shield the electronic spins against magnetic field fluctuations. The analysis of the microwave transmission shows that the spin-photon coupling also becomes maximum at these transitions. The results show that engineering spin-clock states of molecular systems offers a promising strategy to combine sizable spin-photon interactions with a sufficient isolation from unwanted magnetic noise sources

Spin Physik und Cavity-QED mit Stickstoff-Fehlstellen-Zentren in Diamant'

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersIn den letzten Jahren konnte das negativ geladene Stickstoff-Fehlstellen-Zentrum (NV) in Diamant aufgrund seiner vielzähligen Anwendungsmöglichkeiten große Aufmerksamkeit auf sich ziehen. So zum Beispiel reichen die Möglichkeiten von hoch präzisen Sensoren bis hin zu Quanten Informationsprotokollen. Eigenschaften wie lange Spin-Phasen Kohärenz und Lebenszeit stellen das NV Zentrum auch in den Fokus aktueller Forschung. Um all diese Eigenschaften noch besser nutzen zu können, ist es wichtig die zugrundeliegenden physikalischen Phänomene dieses Systems zu verstehen. Ein wichtiger Punkt dabei ist die Wechselwirkung des Spin Freiheitsgrades mit den Phononen des Diamantgitters. In einem Festkörpers sind longitudinale Relaxationsprozesse bei denen der Spin Energie an die Phononen abgibt dominant. Diese Arbeit beschäftigt sich mit der experimentellen Bestimmung der longitudinalen Spin-Gitter Relaxationszeit im Limit von niedrigen Temperaturen. Das Experiment basiert auf Cavity-Quantenelektrodynamik im starken dispersiven Limit. Dort ist es möglich den Spin-Inversionszustand von bis zu 1e16 NV Spins zu bestimmen. Bemerkenswert, die niedrige phononische Zustandsdichte erlaubt der Spininversion über lange Zeitskalen von bis zu 8h zu bestehen. Dieses experimentelle Ergebnis wird zusätzlich durch eine ab initio Simulation basierend auf der Dichtefunktionaltheorie (DFT) gestützt. Es zeigt sich, dass im Niedrigtemperaturlimit ein Einzel-Phononen Prozess den dominanten Realxationsmechanismus darstellt. In einem weiteren Experiment im dispersiven Limit wird die Eignung von NV Zentren für Quanten Informationsaufgaben gezeigt. Zwei räumlich getrennte Spin Ensembles werden an eine gemeinsame bosonische Resonatormode gekoppelt. Diese kohärente Kopplung ist ein erster Schritt in Richtung Verschränkung makroskopischer Ensembles und eröffnet Möglichkeiten ungewöhnliche Dynamik wie zum Beispiel die Relaxation zu negativen Temperaturen zu studieren. Die Kopplung der entfernten Ensembles entsteht durch virtuelle Photonen in der Resonantormode. Kohärente Kopplung ist eine notwendige jedoch nicht hinreichende Bedingung für Quantenverschränkung - ein Phänomen welch experimenteller Nachweis immer noch eine große experimentelle Herausforderung darstellt.Over the last decade the negatively charged nitrogen vacancy (NV) centre in diamond has attracted significant attention for a vast variety of applications ranging from high precision sensing to quantum information tasks. Properties like long phase coherence and spin life-times together with optical addressability have put this solid state spin in the focus of many different types of research. To further exploit all its features it is necessary to understand fundamental physical properties like the interaction of the spin degree of freedom with the diamond lattice phonons. In the solid state environment, the most fundamental process by which an excited spin transfers energy to its surrounding is governed by longitudinal relaxation processes. These processes are usually driven by spin-phonon interaction. This work presents an experimental study of phonon induced longitudinal spin relaxation in the low temperature limit, where quantum effects become relevant. The experiment is based on cavity quantum electrodynamics in the strong dispersive limit. A quantum non-demolition detection scheme is used to read out the inversion state of up to 1e16 NV spins. Remarkably, the main experimental findings show that the low phononic density of states at the NV transition energy enables a non-equilibrium inversion to survive over macroscopic time-scales of up to 8h. Additionally, with an ab initio calculation based on density functional theory it is possible to identify a single phonon process as the main mechanism of spin lattice relaxation in this type of system in the low temperature limit. In a second experiment the possibility to use the NV centre as a building block for quantum information tasks is demonstrated. There, two spatially separated ensembles are coupled to a common bosonic resonator mode. This presents a step towards entanglement of macroscopic spin ensembles and the study of unusual dynamics like negative temperature relaxation. Again, the dispersive regime of cavity QED is utilized and a transverse coupling of the spin ensembles via virtual photons in the resonator is shown. Although it is a fundamental necessity to have coherent coupling between the ensembles, it still remains challenging to experimentally show entanglement in such a system of two distinct spin domains.12